1. Field of the Invention
The invention relates to a multilayered, biaxially oriented, food casing made of thermoplastics, and to processes for its production and use.
2. Description Related Art
Food casings of the type mentioned above are used, in particular, for producing cooked-meat sausage and scalded-emulsion sausage. In general, the longest possible storage life of the sausage is sought. Casings have been developed which have a decreased permeability to water vapor and/or oxygen. A water vapor barrier prevents drying out during storage and an oxygen barrier prevents oxidation of the sausage emulsion. An increasing gray coloration of the emulsion surface indicates oxidation which causes the gradual spoilage of the sausage.
Sausages which are to be stored for longer than three months must typically be sterilized after stuffing. This sterilization is customarily carried out in a steam autoclave at a temperature of about 115 to 130.degree. C. Even with sausage casings having an increased oxygen barrier, a sterilization is required, since in addition to aerobes, anaerobic microorganisms can also lead to spoilage. An ingress of atmospheric oxygen can obviously not be decreased by the sterilization.
A single-layer casing having a good water vapor and oxygen barrier may be produced from vinylidene chloride (VDC) copolymers. However, these special VDC copolymers are expensive and poorly environmentally compatible due to their chlorine content. Therefore, chlorine-free multilayer food casings have been developed which include at least one polymer layer that assures the water vapor barrier or oxygen barrier. These casings are generally produced by coextrusion. EP-A 0 467 039, for example, discloses a tubular casing which consists of an outer layer of aliphatic polyamide, a central layer of polyolefin and an adhesion promotor, and an inner layer of aliphatic and/or partly aromatic (co)polyamides. For sausage goods which are to be stored for a relatively long period, however, the oxygen barrier itself of this casing is typically not sufficient.
Biaxially oriented tubular films having an outer layer of aliphatic polyamide, a central oxygen barrier layer of an ethylene/vinyl alcohol copolymer (EVOH) or of (partly) aromatic copolyamide and an inner layer of olefinic (co)polymers are described in EP-A 0 530 539. However, the polyolefin inner layer leads to an unsatisfactory adhesion of the casing to the sausage emulsion. In order to improve this adhesion, polyolefins which are modified with polar groups can be used for the inner layer. It is also possible to blend the polyolefins with polar (co)polymers. However, all these measures have the disadvantage that they decrease the water-vapor barrier of the inner layer. A corona treatment of the olefinic inner layer likewise leads to an improved emulsion adhesion, but again this is technically very complex.
A further disadvantage of the tubular films as described in EP-A 0 530 539 is that the outer polyamide layer is water-vapor permeable, so that the central EVOH layer can absorb moisture. It is generally known that the oxygen permeation of the EVOH layer increases five hundred to one thousand times if the relative humidity increases from 0 to 100% (see J. Rellmann, H. Schenck in: Kunststoffe 82 [1992] 731). In the case of relatively long storage, in particular in a moist environment, the oxygen barrier of this casing is scarcely greater than that of a single-layer polyamide casing. If the central layer consists of a (partly) aromatic copolyamide (e.g. poly[meta-xylylene adipamide]=nylon MXD6), its oxygen barrier action is retained if the relative humidity is increased from 0 to 100%; however the barrier action is too small in any case. A layer of nylon MDX6 still allows the passage of a tenth of the oxygen volume that penetrates a corresponding layer of nylon 6. In order to prevent oxidation during storage for several months, the oxygen barrier is not sufficient.
EP-A 0 530 549 likewise discloses a three-layer casing having a central EVOH layer. The outer layer here consists of olefinic (co)polymers, while the inner layer is formed of aliphatic polyamide. This casing also does not have a long-lasting oxygen barrier when it is filled with sausage emulsion. The moisture present in abundance in the emulsion slowly migrates through the inner polyamide layer into the central EVOH layer and greatly reduces its oxygen barrier action.
In order to rectify these defects, casings having still more layers have been developed. For example, EP-A 0 603 676 describes a biaxially oriented five-layer tubular film. The layer sequence in this casing is symmetrical. Adjacent to each side of the central EVOH layer are layers of polyolefin, which are each in turn followed by one layer of aliphatic polyamide. The polyolefinic interlayers substantially protect the central layer from moisture. However, the protective action of the interlayers is generally not sufficient against the superheated steam used during the sterilization (see B. C. Tsai and J. A. Wachtel in ACS Symp. Ser. 423 [1990] 198). Although, after the sterilization, the moisture slowly diffuses again out of the casing, a residual moisture remains in the central layer, which greatly reduces the oxygen barrier.
To date, there is no chlorine-free polymer-based tubular food casing which has a high oxygen barrier which remains under all conditions of practice.